Internal combustion engines, including diesel engines, gasoline engines, gaseous fuel-powered engines, and other engines known in the art, may exhaust a complex mixture of air pollutants. The air pollutants may be composed of gaseous and solid compounds, including particulate matter, nitrogen oxides (NOx), and sulfur compounds. Due to heightened environmental concerns, exhaust emission standards have become increasingly stringent. The amount of pollutants emitted from an engine may be regulated depending on the type, size, and/or class of engine. One method that has been implemented by engine manufacturers to comply with the regulation of NOx exhausted to the environment has been to implement a strategy called selective catalytic reduction (SCR).
SCR is a process by which gaseous or liquid reductant (e.g., urea or ammonia) is added to the flow of exhaust from an engine. The combined flow is then absorbed onto a catalyst. The reductant reacts with NOx in the flow of exhaust to form H2O and N2. SCR may be more effective when a ratio of NO to NO2 in the flow of exhaust supplied to the SCR catalyst is about 50:50. Some engines, however, may produce a flow of exhaust having a NO to NO2 ratio of approximately 95:5. In order to increase the relative amount of NO2 to achieve a NO to NO2 ratio of closer 50:50, a diesel oxidation catalyst (DOC) may be located upstream of the SCR catalyst to convert NO to NO2.
One system that includes a DOC to increase a relative amount of NO2 in a flow of exhaust is described in U.S. Pat. No. 6,846,464 (the '464 patent) issued to Montreuil et al. The '464 patent describes a catalytic device including two chambers. The first chamber includes tubes coated with a catalytic material such as platinum that oxidizes NO and hydrocarbons. The second chamber includes tubes coated with a catalytic material such as palladium that oxidizes NO and hydrocarbons. An SCR catalyst is provided downstream from the two chambers of the catalytic device.
Although the system of the '464 patent may provide an oxidation catalyst that increases the amount of NO2 in the flow of exhaust, all of the tubes of the oxidation catalyst are coated with an NO oxidizing material, such as platinum or palladium. Therefore, the entire flow of exhaust contacts either the platinum or the palladium coating on the oxidation catalyst. As a result, there is a risk of providing too much NO2 compared to NO. When there is too much NO2, NOx reduction in the reduction catalyst is much slower, and therefore, a larger reduction catalyst is necessary to effectively reduce NOx in the flow of exhaust.
The disclosed system is directed to overcoming one or more of the problems set forth above.